The objectives and methods of the proposed research are: (1) To use the energy-nitrogen balance procedure and a variety of indexes of protein status (e.g., 3-methylhistidine exretion) to identify with greater certainty the diet (or diets) for weight reduction that permits the highest possible rate of fat loss compatible with conservation of body protein and freedom from functional of structural damage to organs such as the heart and liver; (2) To ascertain whether, like certain genetically obese rodents, men and women with hyperplastic obesity will respond to weight reduction differently from persons with hypertrophic obesity, both metabolically (e.g., in terms of insulin and glucagon response, etc.) and with respect to composition of weight loss; (3) To conduct prospective studies of changes in both composition (see methods below) of severely obese individuals during treatment-induced weight loss and then, after marked weight reduction, to continue to monitor their body composition. These observation will be made in order to understand better the changes in LBM and total body fat that might occur: (a) in patients who are able to maintain weight loss for prolonged periods, and (b) in patients who regain their lost weight despite attempts at weight maintenance. In the latter instance, the changes in body composition that accompany the "relapse" would be monitored; (4) To extend our ongoing studies that involve comparison of various methods for estimating body composition and its changes during weight, loss, weight regain, and weight gain, in order to determine whether newer techniques, such as the use of ultrasound or an electromagnetic measurement will be sufficiently advantageous in convenience, efficiency and cost-effectiveness in relation to their accuracy and sensitivity, to justify their replacing the currently more established methods such as total body potassium by whole body 40K counting, hydrostatic weighing, total body water by tritium dilution, and estimation of total body fat from skinfold thickness at various sites.